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tach signal and F-V converter

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joe_rocket

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I have this circuit, it seems like i can get it working then it stops working. I've changed all sorts of stuff except the tach wire capacitor size. Can somebody please take a look at this and see if it is OK and just that capacitor is wrong or if I have a serious flaw in the wiring (yes, it has two variable resistors on the same line... I just copied what I currently have.. which is not working).
**broken link removed**
 
Read my postings to this thread Your interface to the coil is likely blowing up the FtoV converter.

You don't need the diode; you just need to use the 500Ω pot to set the full scale input voltage for the LM3914
 
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according to the datasheet that is how it is set, but does not specify the capacitor size.
surely the datasheet is correct?
 
according to the datasheet that is how it is set, but does not specify the capacitor size.
surely the datasheet is correct?
So what are you doing about the +-200V that appears across the points?
 
nothing? It works if I disconnect the two chips at the diode junction (and the 100k resistor on pin 5 lm2917 chip). as I rev the motor the voltage increases.
 
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You are using the wrong IC. Your IC is made for a modern variable reluctance pickup that swings above and below the ground reference on pin 8.
The 14 pins version has the inverting input of the opamp available for you to bias it to a positive voltage for old fashioned distributor points.

You have the opamp set for a max input voltage of 11.67V with the 10k and 1.2k resistors to light the top LED but the max output from the tachometer IC is much less so the top few LEDs will never light.
 
You have a 100K feeding through a transistor into a 500 ohm or less load. Think about it.
Also you have no snubber or by pass capacitors on the power side of the system.
The 4.7uf capacitor is unnecessary on the tach input but a back top back 5 - 15 volt zener diode peak clipping circuit parallel to the 18 K and .02 uf capacitor on the input would be beneficial.
 
so you are saying the 100k resistor going into pin #5 is not letting enough current through the 1st circuit to power the second circuit, that's why it powers my meter but not the second circuit? (both work independent of each other). So maybe the pin 5 needs a full hot? also there is a small capacitor between the hot and ground (it is ceramic and says "104" on it...)...
 
You have the opamp set for a max input voltage of 11.67V with the 10k and 1.2k resistors to light the top LED but the max output from the tachometer IC is much less so the top few LEDs will never light.

if I remove the POT from the lm3914 circuit every light lights. also I can make them all light with under 2.0v input...?
 
my base output is around 6v on the 2917, is it ok to trim it's voltage with a POT down to .08v?

if not how do I set the two circuits to match?

(ie, can I set the 3914 to start reading at 6v and increase one led for every 1/10v?
if not how do I set the 2917 to have a base output voltage at 0v for 0 frequency?
 
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It is explained in the datasheet for the LM3914 IC.
Your 1.2k resistor on the LM3914 from pin 8 to ground has the reference voltage of 1.25v of the IC across it and then the current is 1.25V/1.2k= 1.04mA. The same current is in the 10k resistor from pin 7 to pin 8 so it has 10k x 1.04mA= 10.4V across it. Then pin 7 is 1.25V + 10.4V= 11.7V. Pin 6 is also 11.7V and that is the voltage at the input that turns on the 10th LED.

But the 7.5V zener diode in the LM2917-8 limits its supply voltage then its max output voltage is only about +6.5V.

What is "a base output"?

Do you understand that you have the wrong IC for old distributor points?
 
Do you understand that you have the wrong IC for old distributor points?

no need to be condescending, I do but it's working. at any rate i have ordered 3 of the -14's and they should be here in a day or two.

that is beside the piont IMO, what I have now is one chip (2917) putting out 5+v at 0 frequency (base output) we are obviously not on the same page as far as terminology but I'm sure you knew what I meant...

also, your explanation of the 3914 resistors makes little sense b/c it takes less than 2v to light the entire bar (when bypassing the POT).


would it be possible for you to post a schematic that works and let me look at it.
 
it's working.
You are using the IC that is designed for an AC pickup but you are using points that have a DC output. they are completely different.

I have now is one chip (2917) putting out 5+v at 0 frequency (base output)
The datasheet has a graph that shows an ouput voltage of 0V when the input signal is 0Hz. Yours is wrong.

your explanation of the 3914 resistors makes little sense b/c it takes less than 2v to light the entire bar (when bypassing the POT).
I explained in simple Geek-talk how it works. Yours works differently because maybe it is wired differently.

would it be possible for you to post a schematic that works and let me look at it.
Sorry. this is your simple project not mine.
 
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man, I'm just asking for some help here, 3 weeks ago I didn't even know what a potentiometer was...

I have modified the diagram a bit. also I know what I'm seeing, I have 3 different 2917's from 2 different places and they are all putting out over 0v at 0 frequency.
I have double and triple checked my circuits. now maybe I have three burnt chips, if that's the case i really need some help.

is the 4.7uf capacitor going to the coil the correct size? I cannot afford to burn up 3 more chips.....
 
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oh god, you have given directly from sparkplug terminals?? try through a voltage divider and feed it, or use a pulse transformer to step down the voltage.
 
Have you ever measured the voltage of the Tach output? GM use at least two different output voltages.
 
...

...I cannot afford to burn up 3 more chips.....

I gave you a link to a series of posts which tells you exactly why you are destroying chips. You either didn't understand it, or chose to ignore it.
 
I choose to ignore it for 3 reasons.

1) the datasheet does not say that is how to power it, although I am just going by my understanding of the datasheet?
2) no tach I have ever seen needed a shielded cable to run it
3) b/c you posted a link and didn't elaborate... car guys sure a lot nicer and more helpful than electro-geeks.
 
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